This invention relates to an apparatus for applying liquid developer to the surface of a recording medium moving in a path relative to the apparatus and more particularly relates to a biased development zone in which the liquid developer is applied to an image bearing surface of the recording medium through the establishment of a toner meniscus in a toning gap established between the rotating applicator roll and the moving recording medium.
The art of liquid toning of a previously formed electrographic latent image formed on the image bearing surface of a recording medium with a liquid toner or developer is a well established art. The applicator roll is partly submerged in the liquid developer bath provided in a receptacle and the applicator roll is rotated at a sufficiently high velocity so as to raise liquid developer from the bath in the form of a thin toner film on the surface of the roll due to viscous friction. The moving recording medium is brought into spaced proximity with the upper surface of the applicator roll forming a toning gap. Due to the rotational velocity of the applicator roll and also possibly (1) the application of an electrical field across the toning gap or (2) a bias applied either to a back electrode, positioned opposite of the applicator roll, or a bias applied to the applicator roll or a bias applied to both the back electrode and the applicator roll, the maintenance of a toning meniscus in the toning gap can be assured with the desired density level of toning particles, entrained in the liquid developer, being controlled as applied to the image bearing surface. The bias may as well reduce undesirable background staining of the medium as it passes the developer apparatus, which staining reduces toned image contrast and quality. Examples of this art which are relevant to the instant invention are found in the following patents and publications: U.S. Pat. Nos. 3,203,395; 3,256,855, 3,367,791; 3,560,204; and 4,141,317; Japanese Utility Model Laid Open Publication 56-35634; German Offenlegungsschrift (OLS) 2,238,404; and the Benson Electrostatic Printer/Plotter, Model 9336, manufactured by the Benson, P.O. Box 32059, 2600 Orchard Parkway, San Jose, CA 95152.
Probably the most representative of these references to the instant invention is the German OLS 2,238,404. This reference discloses in FIG. 3 the combination of an applicator roll and backrest electrode to which are applied a bais field and which are maintained in a spaced relation to forming a toning gap therebetween. The rotation of the applicator roll, partly submerged in liquid developer, causes the roll to pick up liquid developer supply a meniscus created in the toning gap and in contact with the moving recording medium. The applicator roll is disclosed as being rotated at high speed in a direction opposite to the direction of medium movement, although reference is made that roll can be rotated in the same direction as the direction of medium movement.
Further, the employment of drying rolls are also disclosed in German OLS 2,238,404 down stream from the applicator roll, which roll is rotated in the same direction as the direction of medium movement. However, from the Japanese Laid Open Publication56-35634, it is known to rotate the drying roll in a direction opposite to medium movement.
While such a known liquid developer apparatus may have good application in applying liquid developer to the image bearing surface of a recording medium in the form of a meniscus in a toning gap between two conductive surfaces, the application is still plagued by certain problems. For expamle, except for rotational velocity adjustment of the applicator roll relative to the viscosity of the liquid developer, it is not always practical to obtain, on a continuous and systematic basis, the same amount of liquid developer on the surface of the applicator roll due to developer viscosity changes or changes in temperature or rotational velocity of the applicator roll. What is desired is some means by which the thin film of liquid developer developed on the applicator roll surface is maintained uniform regardless of changes in the above mentioned parameters.
Another problem is the elimination of background stains when the recording medium is momentarily stopped during the continuous application of liquid developer. While the source of developer may be stopped, the formed meniscus at the toner gap will stell remain for a short period of time, toning the image bearing surface of the stopped medium in the stopped position. What is needed is some means by which good image quality can be maintained during medium movement by good developer application at the toner gap with properly applied potentials and the almost instantaneous removal of the formed meniscus when the medium is stopped.
Still a further problem is the elimination of toner particle smearing at the drying roll which not addressed in the prior art.
Still another problem is the reduction of toner buildup on the surface of the applicator roll due to continuous use which effects the toner drawing capabilities of the roll as well as the density and quality of the toned image.
These problems are substantially eleviated by the liquid developer apparatus of the instant invention.